1. Field of the Invention
The present invention relates to a method for manufacturing a group III nitride compound semiconductor laser diode (LD hereinafter) that emits visible short wave rays in the blue region. In particular, the invention relates to a method for forming mirror facets of the LD for better laser cavity.
2. Description of the Related Art
It has been known that a conventional light-emitting device such as a light-emitting diode (LED) and an LD which emits blue light includes a sapphire substrate and group III nitride compound semiconductor layers such as AlGaInN formed thereon. In order to improve reflection and oscillation efficiency of the LD, vertical and parallel degrees of its mirror facets of laser cavity are required to be high. Cleavage is a method for that purpose. It is, however, difficult to cleave the LD formed on the sapphire substrate, because the LD is made of heterogeneous materials. Namely, axis A of the sapphire substrate and that of GaN semiconductor layers formed thereon are discrete to each other and cleavage plane results in uneven and bumpy surface. Accordingly, dry etching is used as an alternate.
Conventionally, a dry etching process is being used both for forming a window for an electrode to insulate an upper and a lower semiconductor layers and for forming mirror facets of laser cavity at a time. The electrodes are inevitably formed on the same side of a sapphire substrate, because the sapphire substrate has an insulative characteristic and a pair of electrodes are required to be formed on the side of semiconductor layers. One electrode is formed on an uppermost semiconductor layer of the LD and another is in contact with some portion of a lower semiconductor layer through a window formed by dry etching.
Dry etching, however, which proceeds in knife shape or tapered shape raises a problem. So as to obtain high vertical and parallel degrees of the mirror facets, the semiconductor layers have to be etched deeply even after dry etching for forming a window is enough. Namely, the dry etching process have to be proceeded further even after a window with depth enough for an electrode is obtained. Accordingly, the lower layer is etched excessively and becomes thinner. As a result, its resistivity and driving voltage increase and luminous efficiency of the LD drops. Further, since a pair of electrodes are formed after the mirror facets are formed, unnecessary materials attach to the mirror facets of the LD during the process of electrodes formation.